The Role of Zinc in Pig Health, Production and Reproduction

Published on: 3/9/2017
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The trace minerals provide essential nutrients required for various metabolic functions such as growth, development, reproduction and immunity. Even moderate deficiency can adversely effect animal performance. Consequent to urbanization and introduction of exotic breed as well as crossbreeding programme in livestock production, the increasing trends toward confinement of livestock, without access to soil or adequate green forage, increases the importance of meeting dietary mineral requirements.
Zinc (Zn) is a trace element involved in various biological functions as compare to other micro-minerals. It is very important multifunctional element required for pig health, production and reproduction. Among the essential macro- and micro-minerals, zinc (Zn) is required for synthesis and metabolic functioning of nearly 300 biochemical enzymes and is considered essential for cell division and synthesis of DNA and protein (Bhowmik et al. 2010) synthesis. Besides, Zn is not widely stored in the body and requires a continuous supply along with the diet (Lonnerdal 2000) specially in pigs as because pig diet is mainly composed of cereal based which contain high amount of phytase that bind with Zn and form insoluble mineral chelate that is not able to absorbed in intestine. Further presence of higher quantity of other inorganic elements like Fe, Cu and Ca also block absorption of Zn in intestine (Krebs 2000).
Recent studies suggest that, Zn may be applied at a pharmacological dose to reduce the stress of weaning. As weaning of piglets is the period of transition from maternal milk and dependency on the sow, to a physically and chemically different diet as well as different feeding regime and environmental stress which are responsible for profound changes in the gastrointestinal tract of the piglets. Now-a-days supplementation of Zn at a pharmacological dose (2000-3000ppm) in the form of zinc oxide is practiced in nursery pigs that helps in  decrease the incidence of post-weaning scouring , increase average daily body weight gain (Case and Carlson, 2002) and also developed better disease resistance capacity. Therefore, the researcher has paid more attention in mineral nutrition especially on Zn. The following paragraphs specifically describe the role of Zn biological system in special reference to swine health.
The significance of zinc in biological system:
Zinc is a structural component of a great number of proteins, including enzymes of cellular signaling pathways, and transcription factors. Zinc can modulate cellular signal recognition, second messenger metabolism, protein kinase, and protein phosphatase activities. In addition to calcium, phosphorus and magnesium, zinc is also important for bone formation as the study revealed that deficiency of Zn reduces the size and strength of femur bone. It is essential for cell proliferation and differentiation, especially for the regulation of DNA synthesis and mitosis (Miller et al. 1968). Zn plays a vital role in maintenance of genomic stability, genetic expression, apoptosis modulation (Almendro et al., 2011). Zn is integral part of DNA repair protein OGG1 which repairs oxidized guanine in DNA. It’s dysregulation leads to point mutations and down regulation in gene expression (Thomas et al., 2015). 
Pharmacological role of zinc:
Recent data suggest that high levels of zinc in certain diets may improve animal health independent of its role on the immune system. Research conducted by Hahn and Baker (1993), Carlson et al. (1999) and Hill et al. (2000) showed that feeding 3,000 ppm zinc, added as zinc oxide, enhances growth and health of nursery pigs. More recently the pharmacological role of zinc as a feed additive for nursery pigs has been demonstrated. Using a trace mineralized salt that is well fortified with bioavailable zinc is the foundation for maintaining the performance, health and vitality of pigs.
Dietary factors affecting Zinc absorption:
- Feed stuff
- Phytate (calcium-phytate-zinc complex) 
- Mainly hexa- and pentaphosphate derivatives
- Highly dependent on calcium
- Amino Acids: histidine and cysteine
- Presence/Absence of other divalent cations: Fe and Ca
- Efficiency of absorption can vary from 15-60% 
- Under normal conditions 1/3 of dietary Zn is absorbed
- Zn status alters efficiency of absorption
- Uptake and retention is greater in growing animals.
Role of Zinc on gastrointestinal tract of pig:
Zinc is associated with the maintenance of function gastrointestinal tract from taste buds of tongue (Berger, 2002) to the function of villus and crypt in the intestine. As reports indicated that gustin the enzyme required for proper development and functioning of taste buds is dependent of Zn (Henkin et al.1999). In intestinal tract Zn helps in maintaining the stability of the intestinal microflora, to support a large diversity of coliforms in weaned piglets (Katouli et al., 1999), and to reduce the susceptibility of pigs to E. coli infection (Mores et al., 1998). Further study has also reported that supplementation of pharmacological doses of Zn as zinc oxide act as an antimicrobial agent (Cromwell, 2001) and improve the gastrointestinal tract function by increasing mucosal thickness, villi height, and width of the small intestine (Li et al., 2001). It was also observed that high dietary zinc (2500 ppm) increase the activity of enzymes viz- amylase, carboxypeptidase A, chymotrypsin, trypsin and lipase in the pancreatic tissue of pigs.
Role of Zn on enzyme activity in pig:
The role of Zn on serum alkaline phosphatase (ALP), glutatamate oxaloacetate transaminase (GOT), glutamate pyruvate transaminase (GPT), erythrocyte Cu Zn superoxide dismutase (Cu/Zn SOD) was studied detail in Zn deficient and supplemted pigs (Borah et al, 2012). Reports of Vergnes et al. 1990, Petkevicius et al. 2003, Sidhuet al. 2005 indicated that the activity of  serum level of ALP, GOT, GPT were altered significantly with the degree of Zn deficiency and the level were returned to normal with supplementation in different species of animals including pigs. Apart from these the enzyme Cu/Zn SOD has a vital role in normal body function as the in mammalian tissues, reactive oxygen species (ROS), such as superoxide radicals (O2-), hydroxyl radicals (OH-) and hydrogen peroxide (H2O2) are continuously generated during aerobic metabolism. Excessive generation of reactive oxygen species can cause detrimental changes, such as lipid peroxidation, DNA breakage, protein degradation, and enzyme inactivation (Dennery, 2007) that lead to cellular death. Therefore these free radicles should be elemented from body. Among the other enzymes that detoxify the free radicles Cu/Zn SOD is one the most important enzyme that catalyzes the elimination of free radicle, further as the name itself indicated it is one of the Zn dependent enzymes. Studied in different species of laboratory animals viz- rat (Dimitrova et al., 2005; Ming et al., 2007; Dimitrova et al., 2008), rhesus macaques (Olin et al., 1995) and human Mariani et al., 2008 indicated the harmful effect of Zn deficiency through free radicles. Similar effect of Zn deficiency was also observed in other experiments (Feng et al. 2007).
Role of Zn on metabolic hormone (tri-iodothyronine and thyroxin) activity in pig:
Normal thyroid status is dependent on the presence of many trace elements for both the synthesis and metabolism of thyroid hormones. The role of zinc in thyroid metabolism has been investigated in animals but with conflicting results (Arthur and Beckett 1999; Baltaci et al. 2004). However, findings of Borah et al., 2014 indicated a higher and lower, tri-iodothyronine and thyroxin concentration in Zn supplemented and Zn deficient pigs, respectively and they concluded that that might be due to the fact that Zn is associated with maintaining the normal physiological status of the thyroid gland (Hartoma et al. 1979) and thyroid follicles (Gupta et al. 1997).
Role of Zn on growth:  
Dietary Zn enhances growth in livestock by activating the various enzyme systems that are essential for cell division and proliferation (Mac Donald, 2000).
Figure 1: Effects of zinc deficiency on metabolic processes associated with growth. (c.f. “The role of zinc in growth and cell proliferation” by MacDonald, R S. in J. Nutr. 130: 1500-1508, 2000).
Impairment of growth due to Zn deficiency observed to be started from decreased taste acuity (hypogeusia) in experimental animals as the symptom of off fed could reversed by the administration of Zn (Tomita, 1977; Tomoko et al.,2001). However, Neto et al. (1995) divided the actions of zinc into following three distinct types in respect of its involvement in growth and development of tissues: 
1) Action on taste and smell acuity, appetite regulation, and food consumption and regulation; 
2) Action on hormonal mediation by participating in
 - GH synthesis and secretion is somatomammotroph cells,
 - The action of GH on liver somatomedin-C production, and 
 - Somatomedin-C activation in bone cartilage. In addition to these multiple functions, zinc also interacts with other hormones somehow related to bone growth such as testosterone, thyroid hormones, insulin, and vitamin D3.
3) Action on DNA and RNA synthesis stimulating 
 - Cell replication and differentiation of chondrocytes, osteoblasts and fibroblasts; 
 - Cell transcription culminating in the synthesis of somatomedin-C (liver), alkaline phosphatase, collagen and osteocalcin (bone), and 
 - protein, carbohydrate and lipid metabolism, that is intimately related to the mechanisms of smell, taste, appetite, and food consumption and utilization; 
On the basis of the above considerations, they conclude that the integration of these mechanisms contributes to the perfect physiological functioning of bone as well overall growth.
Role of Zn on pig reproduction:
Reproduction is an extremely important trait in pig production. To maintain a high reproductive performance breeding animals should reach puberty at an appropriate age or weight, that they are bred successfully, that gilts and sows give birth to large litters with well-developed and uniform piglets with good survivability, that they produce enough milk to support rapid piglet growth during the suckling period and then, when the piglets are weaned, that the sow returns into oestrus within a short time-period.
In case of male animals Zn plays an important role in spermatogenesis as reported studied indicated that in zinc-deficient animals Leydig cell development may be retarded and the response to LH as well as testicular steroidogenesis reduced. Seminal plasma is most important for progressive motility of spermatozoa (Rodriquez-Martinez et al., 1990) and might be of importance to protect membranes of sperm cells and maintain fertilizing capacity during storage (Harrison et al., 1978). Sperm function is highly dependent on ionic environment (Hamamah and Gatti1998). Zinc is one of the most important predominant ions in the seminal plasma. Wong et al. (2002) reported that zinc influences the process of spermatogenesis, sperm motility (Wroblewski et al., 2003), stabilizes sperm membrane (Lewis-Jones et al., 1996), exerts protective, antioxidant-like aktivity (Gavellaand Lipovac 1998), preserves the ability of sperm nuclear chromatin to undergo decondensation during the time of fertilization (Suruki et al., 1995). Moreover, proteins binding Zn ions in boar seminal plasma can presumably protect the sperm plasma membrane against cold shock and stabilize spermatozoa acrosome (Mogielnicka-Brzozowska et al., 2011).
Sow is a polyoestrous breeder and the normal length of an oestrous cycle is 21 days. The oestruous cycle can be divided into a follicular phase, during which follicles grow and mature in the ovary, and a luteal phase, following ovulation and characterised by development of corpora lutea. The control of Zn on estrus behavior, length of estrus cycle and age at puberty is well described (Borah et al.,2014). However, role of Zn on reproductive problems in animals like delayedpuberty and lower conception rates, failure ofimplantation and reduction of litter size (Kreplin, 1992) are also reported. Zinc has a significant role in repair and maintenance of uterine lining following parturition and early return of post-partum estrus (Green et al.,1998). Zn deficient animals have been shown to have lower concentrations of FSH and LH(Boland, 2003). 
Role of Zn on immune system:
Many researchers have assumed that the decreased immune response is a secondary response associated with reduced nutrient intake. Deficiency of Zn causes decreased immunity and loss of T-cell function in animals. Zinc plays a very important role in controlling the immune system, and zinc-deficient animals experience increased susceptibility to a variety of pathogens. The immunologic mechanisms where by zinc modulates increased susceptibility to infection have been studied for several decades. It is clear that zinc affects multiple aspects of the immune system, from the barrier of the skin to gene regulation within lymphocytes. Zinc is crucial for normal development and function of cells mediating non- specific immunity such as neutrophils and natural killer cells. Zinc deficiency also affects development of acquired immunity by preventing both the outgrowth and certain functions of T lymphocytes such as activation of Th1 cytokine production, and B lymphocyte. Likewise, B lymphocyte development and antibody production, particularly immunoglobulin G, is also compromised. The absolute number of macrophage is adversely affected by zinc deficiencies, which can interfere the regulation of intracellular killing, cytokine production, and phagocytosis. The effects of zinc on these key immunologic mediators is rooted in the myriad roles for zinc in basic cellular functions such as DNA replication, RNA transcription, cell division, and cell activation (Shankar and Prasad,1998).
Thymic atrophy, lymphopenia, and compromised cell- and antibody-mediated responses that cause increased rates of infections of longer duration are the immunological hallmarks of zinc deficiency in humans and higher animals. As the deficiency advances, a reprogramming of the immune system occurs, beginning with the activation of the stress axis and chronic production of glucocorticoids that accelerate apoptosis among pre-B and -T cells. This reduces lymphopoiesis and causes atrophy of the thymus. In contrast, myelopoiesis is preserved, thereby providing protection for the first line of immune defense or innate immunity. Changes in gene expression for cytokines, DNA repair enzymes, zinc transporters, signaling molecules, etc., suggest that cells of the immune system are attempting to adapt to the stress of suboptimal zinc. Better understanding of the molecular and cellular changes made in response to inadequate zinc might help in the development of immunotherapeutic interventions (Pamela et al., 2004).
Thymus is important in T-cell formation; the effect of zinc on development of immune system has received considerable research attention in recent years (Cossack, 1989). A zinc deficiency induces the following effects on the immune system:
- Induces lymphoid atrophy and decreases in vivo response to many T-dependent antigens (Fraker et al., 1977; Chandra, 1985).
- Reduces the number of IgM and IgG plaque-forming cells per spleen in response to immunization with sheep red blood cells. The zinc deficiency apparently interferes with T-cell helper function, which causes substantial losses in humoral immune capacity (Moulder and Steward, 1989).
- Drastically reduces the concentration of thymic hormone and thymus weight (Golden et al., 1977).
Zinc deficient newborn animals might be affected to a greater degree than mature ones since the young will exhibit the nutritional effects of a nutritional stress earlier, and will not have the ability to prevent diseases from past immune activity (Beach et al., 1982). Most newborn animals have an extensive thymus gland development, which again points to the possible importance of this gland to the health of the young during their early development.
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